An atom consists of a positively charged nucleus and negatively charged electrons moving around the nucleus. Strong attractive forces between the nucleus and the electrons keep the electrons in the atom. Protons, neutrons and electrons are called subatomic particles.
Although we may see an atom represented like this in many books, we must realise it is not drawn to scale. The diameter of the nucleus is approximately 100,000 times smaller than the diameter of the atom.
The diameter of a typical atom is approximately 1 × 10–10 m.
The diameter of the nucleus is thus approximately 1 × 10–15 m
The charge on a proton is 1.602 × 10–19 C (coulomb) of positive charge. The charge on an electron is 1.602 × 10–19 C of negative charge. i.e. the proton and the electron have equal but opposite charges.
| Subatomic Particle | Relative Charge |
| Proton | + 1 |
| Neutron | 0 |
| Electron | – 1 |
Proton Number, Z
The proton number is the number of protons in the nucleus of an atom.
This number is sometimes called the atomic number.
Each atomic element has a unique proton number.
In a neutral atom, there must to be an equal number of electrons and protons. Thus proton number also tells us how many electrons are in the atom. e.g. a neutral carbon atom has 6 protons and 6 electrons.
Nucleon Number, A
The nucleon number is the number of protons and neutrons in the nucleus of an atom.
This number is sometimes called the mass number. The mass of an atom depends on the number of protons and neutrons in the atom – the electrons are considerably smaller in mass and so account for an insignificant part of the mass of an atom.
Nuclide refers to a distinct kind of atom or nucleus characterised by a specific number of protons and neutrons. Atoms can be defined by its nuclide notation. In general this could be:
Where, A is the nucleon number, Z is the proton number and X is the element (as represented by its chemical symbol).
Thus 
represents a common carbon nuclide having 6 protons and 6 neutrons. This atom would also frequently be referred to as carbon-12.
Isotopes
Isotopes are atoms of the same element having the same number of protons, but different numbers of neutrons.
e.g. 
are three isotopes of carbon.
They all have 6 protons and behave the same way chemically. However, they have different numbers of neutrons in their nuclei and thus have different masses. We sometimes refer to them as carbon-12, carbon-13 and carbon-14.
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